As one of the approaches of Intelligent Transport Systems (ITS) promoted by the Ministry of Land, Infrastructure, Transport and Tourism, services for drivers have been considered, such as alerting a driver currently driving a vehicle and providing alert support. In particular, the collision prevention support service will be an important approach to prevent accidents at an intersection where head-on accidents frequently occur. Currently, informing a driver of a situation at an intersection to be a blind spot for the driver using road sensors and image sensors are under review. Furthermore, the image sensors, such as a camera will be more and more downsized and inexpensive in the future. Although they are currently installed only along the express highways and main roads, in the future, it is expected that they will be installed along any road.
The conventional method of presenting a situation at a blind spot to a driver probably includes capturing images of a road to be a blind spot by an in-vehicle camera, distributing the captured video to rear vehicles, and displaying the video from the driver's point of view on the displays of the rear vehicles, based on the distributed video. For example, Patent Reference 1 discloses an apparatus.
The apparatus disclosed in Patent Reference 1 includes: an imaging unit that obtains an image in the traveling direction of an own vehicle; an own-vehicle image correcting unit that corrects the image obtained by the imaging unit in the traveling direction of the own vehicle to an image seen from a driver; a forward vehicle image receiving unit that receives, from a forward vehicle, the image obtained from the forward vehicle in the traveling direction; a distance calculating unit that calculates a distance to an imaging position of the image received from the forward vehicle image receiving unit; a forward vehicle image correcting unit that corrects the image received from the forward vehicle image receiving unit to the image seen from the driver, based on the distance to the forward vehicle that is calculated by the distance calculating unit; and an image superimposing unit that superimposes the image corrected by the forward vehicle image correcting unit on a portion corresponding to a blind spot for the forward vehicle in the image corrected by the own-vehicle image correcting unit to generate a superimposed display image in which the image of the forward vehicle is made transparent.
Furthermore, Non-patent Reference 2 discloses placing cameras at an intersection, performing two-dimensional projective transformation on images obtained from the cameras, and generating a virtual overhead view as if seen from above the intersection.    Patent Reference 1: Japanese Patent No. 3972722    Non-patent Reference 2: Synthesis of overhead view at an intersection using multiple surveillance cameras, University of Tsukuba, Dec. 6, 2006
However, since only the images in a direction of the driver's line of sight can be generated according to the method disclosed in Patent Reference 1, video of a blind spot caused by an intersection cannot be generated. Even when cameras are installed not inside vehicles but at an intersection as described in Patent Reference 1, a large number of cameras are necessary. Furthermore, each vehicle needs a Global Position System (GPS) receiver for sensing position information. Even when the vehicle is equipped with the GPS receiver, the precision is low and the receiver cannot obtain height information. Thus, there is a problem that the transformed video may become the one captured at a viewpoint far distant from the driver's viewpoint. Furthermore, the method disclosed in Non-patent Reference 1 results in another problem that only an overhead view from above an intersection can be generated, and that it is difficult to make an intuitive determination from which direction a vehicle or a pedestrian enters the intersection at first glance of images from intersection information.